1. Field of the Invention
The present invention relates to an optical head for recording information on a recording carrier and playing back the recorded information by optical means.
2. Description of the Prior Art
In known optical heads for recording and/or playing back information signals from a recording carrier in disc or drum form, in order to cause a beam spot to accurately follow an information recording track or tracks formed spirally or concentrically on the surface of a disc or formed helically or coaxially on the surface of a drum, the position of the beam spot relative to the information recording track must always be controlled.
The vibration of the disc or drum in the surface direction during its rotation is one cause of disturbance of the ideal positional relation in which the beam spot is positioned on the recording track in accurate alignment therewith and this results in a tracking error, (radial error) causing the beam spot to deviate from the recording track. Another cause is the vibration of the disc or drum in the vertical direction during its rotation and this results in a focus error (vertical error), causing a distortion in the shape of the beam spot.
The tracking error and the focus error are always detected during the recording and playback operations and are fed back for the purpose of correcting the relative positions of the recording track and the beam spot. Therefore, if the detection of these errors is not effected accurately, it is difficult to ensure the ideal positional relation.
The most common method of detecting the tracking error is the far-field method utilizing the diffracted light from the recording track for reasons of high accuracy and simplicity of the head construction and the usual method of detecting the focus error is a method utilizing the astigmatism of the optical system for the same reasons.